Corrosion behaviour of thin electroless Ni–P coatings in 1M KOH or 0.5M H ₂ SO ₄

In previous studies, it was found that the decrease in the thickness of electroless deposited Ni–P alloy coating on a steel substrate leads to improvement of the electrocatalytic properties towards the hydrogen evolution reaction (HER) in alkaline medium (1M KOH). In this regard, the corrosion behaviour of thin alloy coatings (0.4–1.9 µm) with different P contents (3.4 –14.6 wt-%) deposited on steel substrate or on substrate of acrylonitrile–butadiene–styrene (ABS) was investigated by anodic potentiodynamic polarisation and by immersion of the coatings in alkaline or in acidic media. The elemental composition, thickness and morphology of the coatings before and after the corrosion tests are characterised by EDS, XRF and SEM. In an alkaline environment, the anodic behaviour of electroless Ni–Р coatings does not differ significantly from that of electrodeposited Ni. In an acidic environment, corrosion performance increases with increasing P content. The anodic dissolution at low anodic polarisation is suppressed, which exerts a corrosion-protective effect in relation to the steel substrate. At high P content (e.g. 14.6 wt-%), the corrosion rate of alloy coatings immersed in the acidic medium is lower compared to that of electrodeposited Ni.